The prototypic acute-phase protein, CRP is an evolutionary preserved protein present both in mammals and the invertebrate Limulus polyphemus. To further examine the evolutionary origin and the physiological function of CRP, studies with Xenopus system was initiated. CRP was isolated and purified from Xenopus laevis and its NH2-terminus sequence determined. Based on this information and the conserved sequence around the Ca(II)- binding site, an 1.O Kb cDNA clone coding for the entire frog CRP was isolated and sequenced. RNA blot analysis indicated that the frog CRP mRNA is about 1.2 Kb in size and was not detectable during the early developmental stages or under normal condition. The deduced amino acid sequence revealed that the frog CRP precursor consists of 239 amino acids with a leader peptide of 17 amino acids. The frog CRP shares 44%, 41%, 35%, and 23% identity respectively with human, rabbit, mouse and Limulus CRPs. The sequence of the putative phosphocholine-binding site was preserved in all species except that the basic residues (Lys, Arg) thought to provide strong interaction with the phosphate group were absent in the frog CRP. Genomic DNA blot analysis suggested that frog CRP gene consists of a single copy. The structure of the regulatory region is under study.